The mechanism by which capsular polysaccharides inhibit phagocytosis of encapsulated microorganisms is not known. The objective of this proposed research is to define the mechanism by which the capsular polysaccharide of Cryptococcus neoformans is able to inhibit phagocytosis of the yeast by macrophages. Proposed studies will (1) characterize the normal mechanisms that mediate phagocytosis of non-encapsulated isolates of the yeast; (2) analyze mechanisms by which the capsular polysaccharide can block these normal phagocytic mechanisms, and (3) examine the biophysical characteristics of the capsular surface that produce a failure in recognition by macrophages. The experimental approach is largely physiocochemical and attempts to characterize and compare the surfaces that encapsulated and non-encapsulated yeast present to phagocytic cells. The proposed research has several unique aspects. First, the experimental model, the capsular polysaccharide or cryptococus neoformans is unusual because we have isolated a non-capsulated variant of the yeast with surface receptors for the capsular polysaccharide. Experimental variation in polysaccharide concentration allows deposition of various amounts of capsular material on the yeast surface. Second, we have developed a method for preservation of the capsular structure during specimen preparation for electron microscopy. This will enable us to use electron microscopy to determine the spatial relation between the capsular and surface binding sites for serum opsonins. Finally, we will utilize appropriate chemical modification reactions to alter properties of the capsular surface that may mediate inhibition of phagocytosis. These studies should yield new information on the pathogenesis of microorganisms that utilize antiphagocytic surface antigens in the production of disease.